Semiconductor device

ABSTRACT

A collector layer, a base layer, and an emitter layer that are disposed on a substrate form a bipolar transistor. An emitter electrode is in ohmic contact with the emitter layer. The emitter layer has a shape that is long in one direction in plan view. A difference in dimension with respect to a longitudinal direction of the emitter layer between the emitter layer and an ohmic contact interface at which the emitter layer and the emitter electrode are in ohmic contact with each other is larger than a difference in dimension with respect to a width direction of the emitter layer between the emitter layer and the ohmic contact interface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. patent application Ser. No.17/386,462 filed on Jul. 27, 2021, which is a Divisional of U.S. patentapplication Ser. No. 16/436,674 filed on Jun. 10, 2019, which claimsbenefit of priority to Japanese Patent Application No. 2018-114345,filed Jun. 15, 2018, the entire content of each is incorporated hereinby reference.

BACKGROUND Technical Field

The present disclosure relates to a semiconductor device.

Background Art

Heterojunction bipolar transistors (HBTs) are mainly used as activeelements that form a power amplifier module of a mobile terminal asdescribed, for example, in Japanese Unexamined Patent ApplicationPublication No. 2005-101402. Desirable characteristics required for theHBTs are items such as high efficiency, high gain, high output, and highbreakdown voltage. In envelope tracking systems, which have recentlyattracted attention, HBTs that operate at a high collector voltage arerequired. In order to realize high-voltage operation of HBTs, it isnecessary to extend the safe operating area (SOA).

SUMMARY

When a collector voltage of an HBT is increased in a graph showingcollector current-collector voltage characteristics (Ic-Vcecharacteristics), a boundary line (SOA line) between the inside and theoutside of a range of the SOA gradually decreases. According toevaluation experiments conducted by the inventors of the presentapplication, a phenomenon that the SOA line discontinuously decreases ata certain collector voltage was found to occur. The collector voltage atwhich the SOA line discontinuously decreases is referred to as a“transition voltage”.

At an operating voltage that is substantially equal to or higher thanthe transition voltage, the risk that the actual operating range becomesout of the range of the SOA increases when a change in the load occursduring the operation of an HBT. If the operating range is out of therange of the SOA, the HBT may be damaged. It is desirable to extend theSOA by increasing the transition voltage so that the HBT is operated ata high collector voltage without being damaged even if a change in theload occurs.

Accordingly, the present disclosure provides a semiconductor device inwhich the SOA can be extended by increasing the transition voltage.

According to an aspect of the present disclosure, there is provided asemiconductor device including a collector layer, a base layer, and anemitter layer that are disposed on a substrate to form a bipolartransistor; and an emitter electrode that is in ohmic contact with theemitter layer. The emitter layer has a shape that is long in onedirection in plan view. A difference in dimension with respect to alongitudinal direction of the emitter layer between the emitter layerand an ohmic contact interface at which the emitter layer and theemitter electrode are in ohmic contact with each other is larger than adifference in dimension with respect to a width direction of the emitterlayer between the emitter layer and the ohmic contact interface.

According to another aspect of the present disclosure, there is provideda semiconductor device including a collector layer, a base layer, and anemitter layer that are disposed on a substrate to form a bipolartransistor; an emitter electrode that is in ohmic contact with theemitter layer; and an emitter wiring line connected to the emitterelectrode through a contact hole formed in an insulating film. Theemitter layer has a shape that is long in one direction in plan view. Adifference in dimension with respect to a longitudinal direction of theemitter layer between the emitter layer and the contact hole is largerthan a difference in dimension with respect to a width direction of theemitter layer between the emitter layer and the contact hole.

According to still another aspect of the present disclosure, there isprovided a semiconductor device including a collector layer, a baselayer, and an emitter layer that are disposed on a substrate to form abipolar transistor; and an emitter electrode that is in ohmic contactwith the emitter layer. The emitter layer has a shape that is long inone direction in plan view. An ohmic contact interface at which theemitter layer and the emitter electrode are in ohmic contact with eachother has a planar shape in which at least one corner of a rectangle ischamfered.

According to still another aspect of the present disclosure, there isprovided a semiconductor device including a collector layer, a baselayer, and an emitter layer that are disposed on a substrate to form abipolar transistor; an emitter electrode that is in ohmic contact withthe emitter layer; and an emitter wiring line connected to the emitterelectrode through a contact hole formed in an insulating film. Theemitter layer has a shape that is long in one direction in plan view.The contact hole has a planar shape in which at least one corner of arectangle is chamfered.

According to still another aspect of the present disclosure, there isprovided a semiconductor device including a collector layer, a baselayer, and an emitter layer that are disposed on a substrate to form abipolar transistor; an emitter electrode that is in ohmic contact withthe emitter layer; and an emitter wiring line connected to the emitterelectrode through a contact hole formed in an insulating film. Theemitter layer has a shape that is long in one direction. In at least oneend portion of the emitter layer, an emitter access resistance which isan electrical resistance from a junction interface between the emitterlayer and the base layer to the emitter electrode is 5 times or more theemitter access resistance in a central portion of the emitter layer.

The above-described arrangement of the emitter electrode, the shape ofthe emitter electrode, the arrangement of the contact hole for anemitter, and the shape of the contact hole for an emitter enable thetransition voltage to be increased to extend the SOA.

Other features, elements, characteristics and advantages of the presentdisclosure will become more apparent from the following detaileddescription of preferred embodiments of the present disclosure withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an HBT according to a reference example forevaluation experiments;

FIG. 2 is a graph showing actual measurement results of SOA lines ofHBTs;

FIG. 3 is a graph showing actual measurement results of collectorcurrent-base voltage characteristics (Ic-Vb characteristics);

FIG. 4 is a plan view of a semiconductor device according to a firstembodiment;

FIG. 5 is a sectional view taken along dash-dotted line 5-5 in FIG. 4;

FIG. 6 is a sectional view taken along dash-dotted line 6-6 in FIG. 4;

FIG. 7 includes a plan view near an emitter layer of a semiconductordevice according to the first embodiment and a graph showing an exampleof a temperature distribution with respect to a longitudinal directionof the emitter layer during operation;

FIG. 8A is a plan view of emitter layers, ohmic contact interfaces, anda base electrode of an HBT prepared for evaluating a transition voltageVt;

FIG. 8B is a graph showing measurement results of a transition voltageVt;

FIG. 9 is a graph showing measurement results of SOA lines of a samplecorresponding to an HBT according to the first embodiment (FIG. 4) and asample corresponding to an HBT according to the reference example (FIG.1);

FIGS. 10A, 10B, and 10C are plan views each illustrating a positionalrelationship of an emitter layer, an emitter electrode, an ohmic contactinterface, a contact hole, and an emitter wiring line;

FIG. 11 is a graph showing measurement results of transition voltages Vtof samples in which positional relationships of an emitter layer, anemitter wiring line, and an ohmic contact interface are thoseillustrated in FIGS. 10B and 10C;

FIG. 12 is a plan view of a semiconductor device according to amodification of the first embodiment;

FIG. 13 is a sectional view of a semiconductor device according toanother modification of the first embodiment;

FIG. 14A is a plan view of an emitter layer, an emitter electrode, andan emitter wiring line of a semiconductor device according to a secondembodiment;

FIG. 14B is a schematic sectional view taken along dash-dotted line14B-14B in FIG. 14A;

FIG. 15A is a plan view of emitter layers, contact holes, and a baseelectrode of an HBT prepared for evaluating a transition voltage Vt;

FIG. 15B is a graph showing measurement results a transition voltage Vt;

FIG. 16 is a plan view of an emitter layer, an emitter electrode, anohmic contact interface, a contact hole, and an emitter wiring line of asemiconductor device according to a third embodiment;

FIG. 17 is a plan view of a semiconductor device according to a fourthembodiment;

FIG. 18 is a plan view of a semiconductor device according to a fifthembodiment;

FIG. 19 is a plan view of a semiconductor device according to a sixthembodiment;

FIG. 20 is a plan view of a semiconductor device according to a seventhembodiment;

FIG. 21 is a plan view of a semiconductor device according to an eighthembodiment;

FIG. 22 is a plan view of a semiconductor device according to a ninthembodiment;

FIGS. 23A, 23B, and 23C are plan views of an emitter layer, an emitterelectrode, a contact hole, and an ohmic contact interface ofsemiconductor devices according to a tenth embodiment and modificationsof the tenth embodiment;

FIG. 24 is a plan view of a semiconductor device according to aneleventh embodiment; and

FIG. 25 is a plan view of a semiconductor device according to a twelfthembodiment.

DETAILED DESCRIPTION

Prior to descriptions of embodiments, one factor that inhibits extensionof the SOA in a typical HBT will be described with reference to FIGS. 1to 3 on the basis of evaluation experiments conducted by the inventorsof the present disclosure.

FIG. 1 is a plan view of an HBT according to a reference example forevaluation experiments. A sub-collector layer 40 made of a semiconductorhaving conductivity is provided on a surface-layer portion of asubstrate. A collector layer 41 and a base layer 51 are disposed on thesub-collector layer 40. The base layer 51 completely overlaps thecollector layer 41 in plan view, and the collector layer 41 and the baselayer 51 are arranged inside the sub-collector layer 40. An emitterlayer 31 is disposed on the base layer 51. The emitter layer 31 isdisposed inside the base layer 51 in plan view. The collector layer 41,the base layer 51, and the emitter layer 31 form a bipolar transistor,for example, an HBT.

The emitter layer 31 has a planar shape that is long in one direction(the lateral direction in FIG. 1) in plan view. The planar shape of theemitter layer 31 is, for example, a rectangle. An emitter electrode 32is disposed on the emitter layer 31. The emitter electrode 32 is formedof a metal and is in ohmic contact with the emitter layer 31. Theinterface at which the emitter electrode 32 and the emitter layer 31 arein ohmic contact with each other is referred to as an “ohmic contactinterface”. An ohmic contact interface 35 completely overlaps theemitter electrode 32 in plan view. The edge of the ohmic contactinterface 35 is disposed slightly inside the edge of the emitter layer31 so as to maintain a substantially uniform gap between the edge of theohmic contact interface 35 and the edge of the emitter layer 31.

A base electrode 52 is disposed on the base layer 51 and is in ohmiccontact with the base layer 51. In FIG. 1, the base electrode 52 isindicated by hatching. The base electrode 52 includes two base electrodemain portions 52A and a base electrode pad portion 52B. The two baseelectrode main portions 52A are disposed on both sides of the emitterlayer 31 in a width direction and extend in a longitudinal direction ofthe emitter layer 31 in plan view. The base electrode pad portion 52Bconnects the two base electrode main portions 52A to each other outsideone end portion (on the left end in FIG. 1) of the emitter layer 31 inthe longitudinal direction. The base electrode 52 including the baseelectrode main portions 52A and the base electrode pad portion 52Bsurrounds the emitter layer 31 so as to form a U-shape.

Collector electrodes 42 are disposed inside the sub-collector layer 40and on both sides of the collector layer 41. The collector electrodes 42each have a planar shape that is long in a direction parallel to thelongitudinal direction of the emitter layer 31. The collector electrodes42 are connected to the collector layer 41 through the sub-collectorlayer 40.

An insulating film is disposed on the emitter electrode 32, thecollector electrodes 42, and the base electrode 52. An emitter wiringline 34, collector wiring lines 44, and a base wiring line 54 aredisposed on the insulating film so as to overlap the emitter electrode32, the collector electrodes 42, and the base electrode pad portion 52B,respectively, in plan view. The emitter wiring line 34 is connected tothe emitter electrode 32 through a contact hole 33 formed in theinsulating film. The collector wiring lines 44 are connected to thecollector electrodes 42 through contact holes 43 formed in theinsulating film. The base wiring line 54 is connected to the baseelectrode pad portion 52B through a contact hole 53 formed in theinsulating film.

The contact hole 33 for an emitter is disposed inside the emitterelectrode 32 in plan view and has a planar shape that is long in thelongitudinal direction of the emitter layer 31. The contact holes 43 forcollectors are disposed inside the collector electrodes 42 in plan viewand each have a planar shape that is long in the longitudinal directionof the collector electrodes 42. The contact hole 53 for a base is formedat an intersection of the base electrode pad portion 52B and anextension of the emitter layer 31 extending in the longitudinaldirection.

The emitter wiring line 34 extends, in a direction parallel to thelongitudinal direction of the emitter layer 31, from the position atwhich the contact hole 33 is disposed. The base wiring line 54 extends,in a direction opposite to the direction in which the emitter wiringline 34 extends, from the position at which the contact hole 53 isdisposed. Second-layer wiring lines may be disposed on the emitterwiring line 34, the collector wiring lines 44, and the base wiring line54.

The emitter layer 31, the emitter electrode 32, and the contact hole 33are arranged symmetrically with respect to both the longitudinaldirection and the width direction in plan view. The gap between the edgeof the emitter layer 31 and the edge of the emitter electrode 32 issubstantially uniform regardless of the longitudinal direction and thewidth direction. The gap between the edge of the emitter layer 31 andthe edge of the contact hole 33 is also substantially uniform regardlessof the longitudinal direction and the width direction. Herein, the term“substantially uniform” means that the variation in a dimension iswithin the range of the variation in terms of process, for example, therange of the variation is 0.5 μm or less.

In general, the area of the emitter electrode 32 is designed as large aspossible in order to ensure a large region in the emitter layer 31 wherea current flows. For example, the gap between the outer peripheral lineof the emitter layer 31 and the outer peripheral line of the emitterelectrode 32 is designed to be 1 μm or less.

When the HBT illustrated in FIG. 1 forms a monolithic microwaveintegrated circuit (MMIC) element in which power amplifiers areincorporated, a plurality of HBTs are disposed on a single semiconductorsubstrate. The plurality of HBTs are electrically connected togetherthrough the emitter wiring line 34, the collector wiring lines 44, thebase wiring line 54, the second-layer wiring lines, etc. either directlyor indirectly with an element such as a resistor therebetween. Thus, apower-stage or driver-stage power amplifier is formed.

FIG. 2 is a graph showing actual measurement results of SOA lines ofHBTs. The horizontal axis represents a collector voltage Vce in units of“V”, and the vertical axis represents a collector current density Jc inunits of “mA/cm²”. The circles and the triangles in the graph show SOAlines based on actual measurement of samples having different emitterdimensions. The circles and the solid line in the graph in FIG. 2represent actual measurement results of a sample that includes anemitter electrode 32 having a width of 3 μm and a length of 40 μm. Thetriangles and the dashed line in the graph represent actual measurementresults of a sample that includes an emitter electrode 32 having a widthof 3 μm and a length of 20 μm. A region on the low-voltage side of eachof the SOA lines corresponds to the SOA.

The graph shows that when the collector voltage Vce increases from 6 Vto 6.5 V, each of the SOA lines rapidly discontinuously decreases. Thecollector voltage Vce at which the SOA line discontinuously decreasescorresponds to a transition voltage Vt.

In the reference example illustrated in FIGS. 1 and 2, the number of theemitter electrodes 32 is one, and the number of the base electrode mainportions 52A is two. In HBTs having other combinations of the number ofthe emitter electrodes 32 and the number of the base electrode mainportions 52A, the discontinuous decrease is similarly confirmed in theSOA lines. The discontinuous decrease in the SOA lines is also confirmedin, for example, an HBT including one emitter electrode 32 and one baseelectrode main portion 52A, an HBT including two emitter electrodes 32and one base electrode main portion 52A, an HBT including two emitterelectrodes 32 and three base electrode main portions 52A, and an HBTincluding three emitter electrodes 32 and four base electrode mainportions 52A.

FIG. 3 is a graph showing actual measurement results of collectorcurrent-base voltage characteristics (Ic-Vb characteristics). Thehorizontal axis represents a base voltage Vb in arbitrary units, and thevertical axis represents a collector current Ic in arbitrary units. Inthe measurement, the base voltage Vb and the collector current Ic weremeasured while sweeping a base current Ib with a current source. Themeasurement was conducted at a plurality of voltages of collectorvoltage Vce=V1, V2, V3, V4, and V5. Here, the voltage V1 to the voltageV5 have a magnitude relationship of V1<V2<V3<V4<V5.

In a range where the collector current Ic is small, the collectorcurrent Ic monotonically increases with an increase in the base voltageVb, and the slope of the collector current Ic with respect to the basevoltage Vb gradually increases. When the collector current Ic is furtherincreased, a snapback point SB at which the slope of the collectorcurrent Ic with respect to the base voltage Vb is infinite appears. Whenthe collector current Ic is further increased beyond the snapback pointSB, the slope of the collector current Ic with respect to the basevoltage Vb changes to negative, and the base voltage Vb decreases withan increase in the collector current Ic.

When the collector voltage Vce is V4 and V5, a kink K at which thecollector current Ic discontinuously decreases appears after thecollector current Ic passes through the snapback point SB. When thecollector voltage Vce is V1, V2, and V3, which is lower than V4 and V5,the kink K does not appear. The minimum collector voltage Vce at whichthe kink K appears corresponds to the transition voltage Vt (FIG. 2).Herein, the term “kink K” refers to a characteristic region where atemporary increase in the base voltage Vb or a temporary decrease in thecollector current Ic appears in a region where the base voltage Vbdecreases and the collector current Ic increases in the Ic-Vbcharacteristics (refer to FIG. 3).

Next, a description will be made of a reason why the kink K appears in aregion beyond the snapback point SB in the collector current-basevoltage characteristics.

The appearance of the kink K is considered to be due to a thermal orelectrical asymmetry of the HBT. In the inside of the emitter layer 31(FIG. 1), the arrangement of the emitter electrode 32 and the contacthole 33 maintains symmetry. However, the collector electrodes 42, thebase electrode 52, various wiring lines, and the like, which areasymmetrically arranged with respect to the emitter layer 31, aredisposed on the periphery of the emitter layer 31. In addition, when thearrangement of a plurality of HBTs and lead wiring lines, circuitelements, via-holes, and the like on the periphery of the HBTs, all ofwhich form a power-stage or driver-stage power amplifier, is viewed fromabove, thermal and electrical asymmetry factors are present for oneemitter layer 31 that is focused on.

When the collector current increases beyond the snapback point SB, aregion where the emitter current Ie mainly flows is shifted by theasymmetry factors in the longitudinal direction of the emitter layer 31(FIG. 1). The kink K (FIG. 3) is considered to appear as a result of theshift of the region where the emitter current Ie mainly flows. Inembodiments described below, the position of the region where theemitter current Ie mainly flows is unlikely to be affected by asymmetryfactors on the periphery of the emitter layer 31.

First Embodiment

A semiconductor device according to a first embodiment will be describedwith reference to FIGS. 4 to 8B.

FIG. 4 is a plan view of a semiconductor device according to the firstembodiment. Hereinafter, the difference from the plan view (FIG. 1) ofthe semiconductor device according to the reference example will bedescribed, and descriptions of common configurations will be omitted.

In the reference example (FIG. 1), the gap between the edge of theemitter layer 31 and the edge of the emitter electrode 32 (ohmic contactinterface 35) is uniform regardless of the longitudinal direction andthe width direction. Herein, a gap (distance) from an edge of an emitterlayer 31, the edge being located at an end portion in the longitudinaldirection of the emitter layer 31, to an edge of an ohmic contactinterface 35, the edge being located at an end portion in thelongitudinal direction of the ohmic contact interface 35, is referred toas a distance a1 with respect to the longitudinal direction. A gap(distance) from an edge of the emitter layer 31, the edge being parallelto the longitudinal direction of the emitter layer 31, to an edge of theohmic contact interface 35, the edge being parallel to the longitudinaldirection of the ohmic contact interface 35, is referred to as adistance a2 with respect to the width direction. The distance a2 withrespect to the width direction of the emitter layer 31 is substantiallyuniform regardless of the position, and the distance a1 with respect tothe longitudinal direction is also substantially uniform regardless ofthe position. In reality, corners of the rectangles of the emitter layer31 and the ohmic contact interface 35 may be rounded in the productionprocess. In such a case, a longitudinal direction component of adistance from a leading end of the emitter layer 31 in the longitudinaldirection to a leading end of the ohmic contact interface 35 in thelongitudinal direction is defined as the distance a1 with respect to thelongitudinal direction.

The dimension of the emitter layer 31 in the longitudinal direction is,for example, 5 μm or more and 60 μm or less (i.e., from 5 μm to 60 μm).The dimension of the emitter layer 31 in the width direction is, forexample, 1 μm or more and 8 μm or less (i.e., from 1 μm to 8 μm).

In the first embodiment, the distance a1 with respect to thelongitudinal direction of the emitter layer 31 is longer than thedistance a2 with respect to the width direction of the emitter layer 31.As a result, the difference in dimension with respect to thelongitudinal direction between the emitter layer 31 and the ohmiccontact interface 35 (double the distance a1 with respect to thelongitudinal direction) is larger than the difference in dimension withrespect to the width direction between the emitter layer 31 and theohmic contact interface 35 (double the distance a2 with respect to thewidth direction).

FIG. 5 is a sectional view taken along dash-dotted line 5-5 in FIG. 4. Asub-collector layer 40 is disposed on a substrate 60 made of asemi-insulating semiconductor. A collector layer 41 is disposed on apartial region of the sub-collector layer 40, and a base layer 51 isdisposed on the collector layer 41. Edges of the base layer 51 coincidewith edges of the collector layer 41. An emitter layer 31 is disposed ona partial region of the base layer 51. The emitter layer 31 includes,for example, three layers, namely, a narrow-sense emitter layer 31A, acap layer 31B, and a contact layer 31C. The collector layer 41, the baselayer 51, and the emitter layer 31 form an HBT.

Collector electrodes 42 are disposed in regions on both sides of thecollector layer 41 on the upper surface of the sub-collector layer 40.The collector electrodes 42 are connected to the collector layer 41through the sub-collector layer 40. Base electrode main portions 52A aredisposed in regions on both sides of the emitter layer 31 on the uppersurface of the base layer 51. The base electrode main portions 52A arein ohmic contact with the base layer 51. An emitter electrode 32 isdisposed in a partial region on the upper surface of the emitter layer31. The interface between the emitter electrode 32 and the emitter layer31 corresponds to an ohmic contact interface 35.

An insulating film 61 is disposed so as to cover the collectorelectrodes 42, the base electrode main portions 52A, and the emitterelectrode 32. Collector wiring lines 44 and an emitter wiring line 34are disposed on the insulating film 61. The collector wiring lines 44are connected to the collector electrodes 42 through contact holes 43formed in the insulating film 61. The emitter wiring line 34 isconnected to the emitter electrode 32 through a contact hole 33 formedin the insulating film 61.

FIG. 6 is a sectional view taken along dash-dotted line 6-6 in FIG. 4. Asub-collector layer 40, a collector layer 41, a base layer 51, anemitter layer 31, and an emitter electrode 32 are stacked on a substrate60 in this order. A base electrode pad portion 52B is disposed in apartial region on the upper surface of the base layer 51. An insulatingfilm 61 covers the emitter layer 31, the emitter electrode 32, and thebase electrode pad portion 52B. An emitter wiring line 34 and a basewiring line 54 are disposed on the insulating film 61. The emitterwiring line 34 is connected to the emitter electrode 32 through acontact hole 33 formed in the insulating film 61. The base wiring line54 is connected to the base electrode pad portion 52B through a contacthole 53 formed in the insulating film 61. The emitter electrode 32 canbe approximately assumed to be equipotential because the emitterelectrode 32 is formed of a low-resistance material such as a metalhaving electrical conductivity sufficiently higher than that of theemitter layer 31 formed of a semiconductor. In FIG. 6, the emitterelectrode 32 is assumed to be equipotential, and only the resistancefrom the emitter electrode 32 to an emitter-base junction interface isschematically illustrated.

The emitter layer 31 is divided into a region right under an ohmiccontact interface 35 (hereinafter referred to as a “central region 36”)and regions outside the ohmic contact interface 35 (hereinafter referredto as “end regions 37”). In the central region 36, an emitter currentflows in the emitter layer 31 mainly in the thickness direction betweenthe emitter electrode 32 and the base layer 51. In contrast, in the endregions 37, an emitter current flows in the emitter layer 31 from thebase layer 51 not only in the thickness direction but also in thein-plane direction and reaches the emitter electrode 32 because theemitter electrode 32 does not overlap the emitter layer 31. Therefore,an emitter access resistance in the end regions 37 is increased by theamount of resistance corresponding to the sheet resistance of theemitter layer 31 as compared with the central region 36. Herein, theterm “emitter access resistance” means an electrical resistance of acurrent path from the interface between the emitter layer 31 and thebase layer 51 to the interface between the emitter wiring line 34 andthe emitter electrode 32.

In one example, the narrow-sense emitter layer 31A (FIG. 5) is formed ofn-type InGaP having a Si doping concentration of 2×10¹⁷ cm⁻³ or more and5×10¹⁷ cm⁻³ or less (i.e., from 2×10¹⁷ cm⁻³ to 5×10¹⁷ cm⁻³) and has athickness of 20 nm or more and 50 nm or less (i.e., from 20 nm to 50nm). The cap layer 31B (FIG. 5) is formed of n-type GaAs having a Sidoping concentration of 2×10¹⁸ cm⁻³ or more and 4×10¹⁸ cm⁻³ or less(i.e., from 2×10¹⁸ cm⁻³ to 4×10¹⁸ cm⁻³) and has a thickness of 50 nm ormore and 200 nm or less (i.e., from 50 nm to 200 nm). The contact layer31C (FIG. 5) is formed of n-type InGaAs having a Si doping concentrationof 1×10¹⁹ cm⁻³ or more and 3×10¹⁹ cm⁻³ or less (i.e., from 1×10¹⁹ cm⁻³to 3×10¹⁹ cm⁻³) and has a thickness of 100 nm or more and 200 nm or less(i.e., from 100 nm to 200 nm). Accordingly, the increase in the emitteraccess resistance is mainly due to the sheet resistance of the cap layer31B and the contact layer 31C. For example, the total sheet resistanceof the emitter layer 31 including the three layers is 20 Ω/sq. or moreand 50 Ω/sq. or less (i.e., from 20 Ω/sq. to 50 Ω/sq.).

Next, advantageous effects of the first embodiment will be describedwith reference to FIG. 7.

FIG. 7 includes a plan view near an emitter layer 31 of a semiconductordevice according to the first embodiment and a graph showing an exampleof a temperature distribution with respect to the longitudinal directionof the emitter layer 31 during operation. The horizontal axis of thegraph showing the temperature distribution represents a position of theemitter layer 31 in the longitudinal direction, and the vertical axis ofthe graph represents a temperature.

In the end regions 37, when the emitter current increases and exceedsthe snapback point SB (FIG. 3), a voltage drop becomes larger than thatin the central region 36 due to the emitter access resistance. As aresult, the net base-emitter voltage excluding the effect of theparasitic resistance decreases, and an emitter current flowing in theend regions 37 is suppressed. Specifically, in the end regions 37, thedensity of a current flowing through the emitter-base junction interfaceis decreased compared with the central region 36. The decrease in thecurrent density relatively decreases the temperature of each of the endregions 37 compared with the temperature of the central region 36.

The relative decrease in the temperature of each of the end regions 37causes a further relative decrease in the current density. Due to thischain of feedback, in the collector current-base voltage characteristics(Ic-Vb characteristics) shown in FIG. 3, a current flowing in the endregions 37 rapidly decreases in the high-current range after passing ofthe snapback point SB compared with that in the low-current range beforepassing of the snapback point SB. As a result, the current does notsubstantially flow in the end regions 37. A region where the emittercurrent flows mainly and a region having a high temperature aresubstantially limited to the central region 36.

Since the region where the emitter current flows mainly and the regionhaving a high temperature are limited to the central region 36 of theemitter layer 31, the emitter current is unlikely to be affected by thethermal and electrical asymmetries near the end portions of the emitterlayer 31. Accordingly, the appearance of the kink K (FIG. 3) issuppressed to achieve the effect of increasing the transition voltage Vt(FIG. 2). As a result, the range of the SOA is extended, and theoperation at a high collector voltage can be realized.

Next, a preferred distribution of the emitter access resistance will bedescribed. In order to extend the range of the SOA, the emitter accessresistance in at least one end portion of the emitter layer ispreferably larger than the emitter access resistance in a centralportion of the emitter layer. In order to obtain a sufficient effect ofextending the range of the SOA, the emitter access resistance in atleast one end portion of the emitter layer is preferably 5 times or morethe emitter access resistance in the central portion of the emitterlayer. Although it is difficult to actually measure the emitter accessresistance of an HBT, the emitter access resistance can be determinedby, for example, performing a numerical simulation.

Next, a preferred dimension of the distance a1 with respect to thelongitudinal direction of the emitter layer 31 for extending the rangeof the SOA will be described with reference to FIGS. 8A and 8B. Anincrease in the transition voltage Vt (FIG. 2), which is a collectorvoltage Vce at which the SOA line discontinuously decreases, means anextension of the range of the SOA. Therefore, a preferred dimension ofthe distance a1 with respect to the longitudinal direction wasdetermined by evaluating the transition voltage Vt.

FIG. 8A is a plan view of emitter layers 31, ohmic contact interfaces35, and a base electrode 52 of an HBT prepared for evaluating thetransition voltage Vt. A plurality of HBTs having different distances a1with respect to the longitudinal direction of the emitter layer 31 wereactually prepared, and the transition voltage Vt was measured. Thesamples prepared above are so-called double-emitter HBTs in whichemitter layers 31 are disposed on both sides of a base electrode mainportion 52A. Each of the emitter layers 31 has a length of 40 μm and awidth of 3 μm. The distance a1 with respect to the longitudinaldirection in one end portion of the emitter layer 31 is equal to thedistance a1 with respect to the longitudinal direction in the other endportion of the emitter layer 31. The distance a2 with respect to thewidth direction is 0.3 μm.

FIG. 8B is a graph showing measurement results of the transition voltageVt. The horizontal axis represents the distance a1 with respect to thelongitudinal direction of the emitter layer 31 in units of “μm”, and thevertical axis represents the transition voltage Vt in units of “V”. In arange where the distance a1 with respect to the longitudinal directionis 2.2 μm or less, the transition voltage Vt is about 6.3 V. In a rangewhere the distance a1 with respect to the longitudinal direction is 3 μmor more, the transition voltage Vt increases to about 8 V. For example,when the distance a1 with respect to the longitudinal direction isincreased from 2.2 μm to 3.2 μm, the transition voltage Vt increases byabout 1.9 V.

The results of the evaluation experiment in FIG. 8B show that asignificant effect of increasing the transition voltage Vt is obtainedwhen the distance a1 with respect to the longitudinal direction is 3 μmor more. This effect is generated by increasing the emitter accessresistance from the emitter electrode 32 to the emitter-base junctioninterface in an end region 37, as described with reference to FIG. 7.

In order to obtain the effect of increasing the transition voltage Vt,the distance a1 with respect to the longitudinal direction is notnecessarily increased in both end portions of the emitter layer 31. Acertain effect of increasing the transition voltage Vt is obtained aslong as the distance a1 with respect to the longitudinal direction isincreased in at least one end portion of the emitter layer 31. Forexample, the difference in dimension with respect to the longitudinaldirection between the emitter layer 31 and the ohmic contact interface35 (double the distance a1 with respect to the longitudinal direction)is preferably larger than the difference in dimension with respect tothe width direction between the emitter layer 31 and the ohmic contactinterface 35 (double the distance a2 with respect to the widthdirection). In particular, the difference in dimension with respect tothe longitudinal direction between the emitter layer 31 and the ohmiccontact interface 35 is preferably 10 times or more the difference indimension with respect to the width direction between the emitter layer31 and the ohmic contact interface 35. When one end portion of theemitter layer 31 is focused on, the distance a1 with respect to thelongitudinal direction is preferably 5 times or more the difference indimension with respect to the width direction between the emitter layer31 and the ohmic contact interface 35 in at least one end portion.

In order to confirm the extension of the SOA, a sample corresponding tothe HBT according to the first embodiment (FIG. 4) and a samplecorresponding to the HBT according to the reference example (FIG. 1)were prepared, and an evaluation experiment for measuring the SOA linewas conducted. The result of this evaluation experiment will now bedescribed with reference to FIG. 9.

FIG. 9 is a graph showing measurement results of the SOA lines of thesample corresponding to the HBT according to the first embodiment (FIG.4) and the sample corresponding to the HBT according to the referenceexample (FIG. 1). The samples prepared above each have thedouble-emitter structure illustrated in FIG. 8A. In the samplecorresponding to the HBT according to the first embodiment (FIG. 4), thepositional relationship between an emitter layer 31 and an emitterelectrode 32 is the same as that of the HBT according to the firstembodiment. In the sample corresponding to the HBT according to thereference example (FIG. 1), the positional relationship between anemitter layer 31 and an emitter electrode 32 is the same as that of theHBT according to the reference example. The horizontal axis of the graphshown in FIG. 9 represents a collector voltage Vc in units of “V”, andthe vertical axis of the graph represents a collector current Ic inunits of “A”. The solid line and the dashed line in the graph in FIG. 9represent the measurement results of the SOA lines of the samplecorresponding to the HBT according to the first embodiment and thesample corresponding to the HBT according to the reference example,respectively.

The graph shows that a transition voltage Vt1 of the samplecorresponding to the HBT according to the first embodiment is higherthan a transition voltage Vt0 of the sample corresponding to the HBTaccording to the reference example. This evaluation experiment showsthat the range of the SOA is extended by using the structure of the HBTaccording to the first embodiment.

Next, a relative preferred positional relationship between an emitterlayer 31 and an emitter wiring line 34 will be described with referenceto FIGS. 10A to 11.

FIGS. 10A, 10B, and 10C are plan views each illustrating a positionalrelationship of an emitter layer 31, an emitter electrode 32, an ohmiccontact interface 35, a contact hole 33, and an emitter wiring line 34.In each of the examples illustrated in FIGS. 10A, 10B, and 10C, thedistance a1 with respect to the longitudinal direction of the emitterlayer 31 is longer than the distance a2 with respect to the widthdirection of the emitter layer 31.

In the example illustrated in FIG. 10A, each of end portions of theemitter wiring line 34 is disposed outside the corresponding end portionof the emitter layer 31 with respect to the longitudinal direction ofthe emitter layer 31. In the examples illustrated in FIGS. 10B and 10C,each of end portions of the emitter wiring line 34 is disposed betweenthe corresponding end portion of the emitter layer 31 and thecorresponding end portion of the ohmic contact interface 35 with respectto the longitudinal direction of the emitter layer 31. In the exampleillustrated in FIG. 10B, the end portion of the emitter wiring line 34is disposed outside the center between the end portion of the emitterlayer 31 and the end portion of the ohmic contact interface 35 (on theside closer to the end portion of the emitter layer 31). In contrast, inthe example illustrated in FIG. 10C, the end portion of the emitterwiring line 34 is disposed inside the center between the end portion ofthe emitter layer 31 and the end portion of the ohmic contact interface35 (on the side closer to the end portion of the ohmic contact interface35).

FIG. 11 is a graph showing measurement results of transition voltages Vtof samples in which the positional relationships of an emitter layer 31,an emitter wiring line 34, and an ohmic contact interface 35 are thoseillustrated in FIGS. 10B and 10C. The horizontal axis represents thedistance a1 with respect to the longitudinal direction in units of “μm”,and the vertical axis represents the transition voltage Vt in units of“V”. The solid line and the dashed line of the graph in FIG. 11represent the measurement results of the sample corresponding to theembodiment illustrated in FIG. 4 and the sample corresponding theexample illustrated in FIG. 10C, respectively.

The samples actually prepared above each have the double-emitterstructure illustrated in FIG. 8A. In the sample corresponding to theembodiment illustrated in FIG. 4, the layout in the HBT according to theembodiment illustrated in FIG. 4 is used as the layout of the emitterlayer 31, the emitter electrode 32, and the emitter wiring line 34 ineach of the two emitter structures. More specifically, the samplecorresponding to the embodiment illustrated in FIG. 4 corresponds to anexample in which one end portion of the emitter layer 31 has theconfiguration of the sample illustrated in FIG. 10B and the other endportion of the emitter layer 31 has the configuration of the sampleillustrated in FIG. 10A. In the sample corresponding to the exampleillustrated in FIG. 10C, the layout illustrated in FIG. 10C is used asthe layout of the emitter layer 31, the emitter electrode 32, and theemitter wiring line 34 in one end portion of each of the two emitterstructures. In the other end portion, the layout illustrated in FIG. 10Ais used in order to lead the emitter wiring line 34 to the outside.

The graph in FIG. 11 shows that the effect of increasing the transitionvoltage Vt is obtained when an end portion of the emitter wiring line 34is disposed outside the center between an end portion of the emitterlayer 31 and an end portion of the ohmic contact interface 35. Thereason for the increase in the transition voltage Vt will be describedbelow.

In a portion of an end region 37 of the emitter layer 31, the portionoverlapping the emitter wiring line 34 (FIG. 6) in plan view, heat istransferred from the emitter layer 31 through the insulating film 61mainly in the thickness direction and reaches the emitter wiring line34. On the other hand, in a portion of an end region 37 of the emitterlayer 31, the portion not overlapping the emitter wiring line 34 (FIG.6) in plan view, heat is transferred from the emitter layer 31 throughthe insulating film 61 not only in the thickness direction but also inthe lateral direction and reaches the emitter wiring line 34. Thethermal conductivity of the insulating film 61 is lower than the thermalconductivity of the emitter wiring line 34. Therefore, in a portion ofthe end region 37 of the emitter layer 31, the portion not overlappingthe emitter wiring line 34, heat generated in the emitter layer 31 isunlikely to be dissipated. Accordingly, in the case where a largeportion of the end region 37 overlaps the emitter wiring line 34 (FIG.10B), an increase in the temperature of the emitter layer 31 issuppressed compared with the case where a small portion of the endregion 37 overlaps the emitter wiring line 34 (FIG. 10C). Consequently,a current flowing in the end region 37 further decreases. As a result,the current is unlikely to be influenced by the effect of thermal andelectrical asymmetries, and the transition voltage Vt increases.Furthermore, considering the emitter wiring line 34 disposed on theemitter layer 31 and used for leading to the outside, the configurationillustrated in FIG. 10B has a higher thermal symmetry than theconfiguration illustrated in FIG. 10C. Therefore, the effect of thethermal asymmetry is reduced, and the effect of increasing thetransition voltage Vt is further provided.

The results of the evaluation experiment shown in FIG. 11 and theconsiderations described above show that an end portion of the emitterwiring line 34 is preferably disposed outside the center between an endportion of the emitter layer 31 and an end portion of the ohmic contactinterface 35 (including the outside of the end portion of the emitterlayer 31 as illustrated in FIG. 10A).

In one end region 37 of the emitter layer 31, an end portion of theemitter wiring line 34 may be disposed between the corresponding endportion of the emitter layer 31 and the corresponding end portion of theohmic contact interface 35, and the other end region 37 may be disposedsuch that the entire region thereof overlaps the emitter wiring line 34.

Next, a semiconductor device according to a modification of the firstembodiment will be described with reference to FIG. 12.

FIG. 12 is a plan view of a semiconductor device according to amodification of the first embodiment. The semiconductor device accordingto this modification includes a plurality of unit transistors 70. Theunit transistors 70 each have the same configuration as thesemiconductor device (FIGS. 4, 5, and 6) according to the firstembodiment. The plurality of unit transistors 70 are arranged in adirection (in the up-down direction in FIG. 12) orthogonal to thelongitudinal direction of an emitter layer 31.

An emitter wiring line 34 extends from each of the unit transistors 70toward one side (the right side in FIG. 12) in the longitudinaldirection. The emitter wiring lines 34 extending from the unittransistors 70 are continuous with an emitter common wiring line (groundwiring line) 71. A via hole 72 is formed inside the emitter commonwiring line 71 in plan view. The via hole 72 extends through a substrate60 (FIGS. 5 and 6) and reaches a back surface of the substrate 60. Theemitter common wiring line 71 is connected to a ground electrode forexternal connection with a metal member disposed in the via hole 72therebetween, the ground electrode being disposed on the back surface ofthe substrate 60.

A base wiring line 54 extends from each of the unit transistors 70toward a direction (the left side in FIG. 12) opposite to the directionin which the emitter wiring line 34 extends. The width of each of thebase wiring lines 54 is increased, and the base wiring lines 54 overlapa radio-frequency input wiring line 75. Portions where each of the basewiring lines 54 overlaps the radio-frequency input wiring line 75function as a capacitor 76 with an MIM structure. Furthermore, the basewiring lines 54 are each connected to a bias wiring line 78 with athin-film resistance 77 therebetween.

Although not shown in FIG. 12, collector wiring lines 44 of each of theunit transistors 70 are connected to a collector common wiring line(radio-frequency output wiring line) disposed above the emitter commonwiring line 71. The emitter common wiring line 71 and the collectorcommon wiring line may be independently connected to a Cu pillar bump, asolder bump, or the like.

As illustrated in FIG. 12, various wiring lines, circuit elements, thevia hole, etc. are asymmetrically arranged in a lateral direction withrespect to each of the unit transistors 70. Even in the laterallyasymmetric configuration, the transition voltage Vt can be increased toextend the range of the SOA by using, as each of the unit transistors70, the configuration of the semiconductor device according to the firstembodiment.

Next, another modification of the first embodiment will be describedwith reference to FIG. 13.

FIG. 13 is a sectional view of a semiconductor device according to thismodification. In the first embodiment, the emitter wiring line 34 isconnected to the emitter layer 31 with the emitter electrode 32therebetween as illustrated in FIG. 5. In this modification, an emitterwiring line 34 is directly connected to an emitter layer 31 asillustrated in FIG. 13. In this structure, the interface between theemitter wiring line 34 and the emitter layer 31 functions as an ohmiccontact interface 35. A contact hole 33 formed in an insulating film 61and the ohmic contact interface 35 completely overlap in plan view.

In this modification, the transition voltage Vt can be increased toextend the range of the SOA by determining the positional relationshipbetween the emitter layer 31 and the ohmic contact interface 35 as inthe case of the first embodiment.

Second Embodiment

Next, a semiconductor device according to a second embodiment will bedescribed with reference to FIGS. 14A to 15B. Hereinafter, descriptionsof configurations that are common to those of the semiconductor deviceaccording to the first embodiment (FIGS. 4, 5, and 6) will be omitted.

FIG. 14A is a plan view of an emitter layer 31, an emitter electrode 32,and an emitter wiring line 34 of a semiconductor device according to thesecond embodiment. In the first embodiment, the transition voltage Vt isincreased by making the distance a1 with respect to the longitudinaldirection of the emitter layer 31 longer than the distance a2 withrespect to the width direction of the emitter layer 31. A gap (distance)from an edge of an emitter layer 31, the edge being located at an endportion in the longitudinal direction of the emitter layer 31, to anedge of a contact hole 33, the edge being located at an end portion inthe longitudinal direction of the contact hole 33, is referred to as adistance b1. A gap (distance) from an edge of the emitter layer 31, theedge being parallel to the longitudinal direction of the emitter layer31, to an edge of the contact hole 33, the edge being parallel to thelongitudinal direction of the contact hole 33, is referred to as adistance b2. In the second embodiment, the transition voltage Vt isincreased by making the distance b1 with respect to the longitudinaldirection of the emitter layer 31 longer than the distance b2 withrespect to the width direction of the emitter layer 31. The distance a1with respect to the longitudinal direction and the distance a2 withrespect to the width direction are substantially equal to each other. Inone example, the distance a1 with respect to the longitudinal directionand the distance a2 with respect to the width direction are each about0.5 μm or less, and the distance b1 with respect to the longitudinaldirection is 4 μm or more.

FIG. 14B is a schematic sectional view taken along dash-dotted line14B-14B in FIG. 14A. With respect to the longitudinal direction of theemitter layer 31, a region that overlaps the contact hole 33 is definedas a central region 36, and regions outside the contact hole 33 aredefined as end regions 37 in the second embodiment. In the centralregion 36, a current flows from a junction interface between the baselayer 51 and the emitter layer 31 toward the emitter wiring line 34 inthe contact hole 33 through the emitter layer 31 and the emitterelectrode 32 mainly in the thickness direction. In the end regions 37, acurrent flows from the junction interface between the base layer 51 andthe emitter layer 31 through the emitter layer 31 mainly in thethickness direction and then flows in the emitter electrode 32 in thelateral direction.

In the second embodiment, the emitter access resistance is relativelyhigh in the end regions 37 as in the case of the first embodiment.Accordingly, the effect of increasing the transition voltage Vt toextend the range of the SOA is obtained.

Next, a preferred dimension of the distance b1 with respect to thelongitudinal direction of the emitter layer 31 will be described withreference to FIGS. 15A and 15B.

FIG. 15A is a plan view of emitter layers 31, contact holes 33, and abase electrode 52 of an HBT prepared for evaluating the transitionvoltage Vt. A plurality of HBTs having different distances b1 withrespect to the longitudinal direction were actually prepared, and thetransition voltage Vt was measured. The samples prepared above areso-called double-emitter HBTs in which emitter layers 31 are disposed onboth sides of a base electrode main portion 52A. Each of the emitterlayers 31 has a length of 40 μm and a width of 3 μm. The distance b1with respect to the longitudinal direction in one end portion of theemitter layer 31 is equal to the distance b1 with respect to thelongitudinal direction in the other end portion of the emitter layer 31.The distance b2 with respect to the width direction is 0.3 μm.

FIG. 15B is a graph showing measurement results of the transitionvoltage Vt. The horizontal axis represents the distance b1 with respectto the longitudinal direction in units of “μm”, and the vertical axisrepresents the transition voltage Vt in units of “V”. With an increasein the distance b1 with respect to the longitudinal direction from about3 μm to about 10 μm, the transition voltage Vt gradually increases. Forexample, when the distance b1 with respect to the longitudinal directionis 2.5 μm or more and 3.5 μm or less (i.e., from 2.5 μm to 3.5 μm), theincrease in the transition voltage Vt is not observed.

The results of the evaluation experiment in FIG. 15B show that theeffect of increasing the transition voltage Vt is obtained when thedistance b1 with respect to the longitudinal direction is 4 μm or more.When the distance b1 with respect to the longitudinal direction is 7 μmor more, the effect is significantly observed.

In the second embodiment, the increase in the emitter access resistancein the end regions 37 (FIG. 14B) is due to the sheet resistance of theemitter electrode 32 (FIG. 14B). In contrast, in the first embodiment,the increase in the emitter access resistance in the end regions 37 isdue to the sheet resistance of the emitter layer 31 (FIG. 6). The sheetresistance of the emitter electrode 32 (about 0.5 Ω/sq. or more andabout 10 Ω/sq. or less—i.e., from about 0.5 Ω/sq. to about 10 Ω/sq.) islower than the sheet resistance of the emitter layer 31 (about 20 Ω/sq.or more and about 50 Ω/sq. or less—i.e., from about 20 Ω/sq. to about 50Ω/sq.). Therefore, the tendency of the increase in the transitionvoltage Vt in the second embodiment (FIG. 15B) is gentler than thetendency of the increase in the transition voltage Vt in the firstembodiment (FIG. 8B).

In order to obtain the effect of increasing the transition voltage Vt,the distance b1 with respect to the longitudinal direction is notnecessarily increased in both end portions of the emitter layer 31 as inthe case of the first embodiment. A certain effect of increasing thetransition voltage Vt is obtained as long as the distance b1 withrespect to the longitudinal direction is increased in at least one endportion of the emitter layer 31. For example, the difference indimension with respect to the longitudinal direction between the emitterlayer 31 and the contact hole 33 (double the distance b1 with respect tothe longitudinal direction) is preferably larger than the difference indimension with respect to the width direction between the emitter layer31 and the contact hole 33 (double the distance b2 with respect to thewidth direction). In particular, the difference in dimension withrespect to the longitudinal direction between the emitter layer 31 andthe contact hole 33 is preferably 10 times or more the difference indimension with respect to the width direction between the emitter layer31 and the contact hole 33. When one end portion of the emitter layer 31is focused on, the distance b1 with respect to the longitudinaldirection is preferably 5 times or more the difference in dimension withrespect to the width direction between the emitter layer 31 and thecontact hole 33 in at least one end portion.

Next, a relative preferred positional relationship of an emitter layer31, a contact hole 33, and an emitter wiring line 34 will be described.

As in the case where the description has been made in the firstembodiment with reference to FIGS. 10A to 11, an end portion of theemitter wiring line 34 (FIG. 14A) is preferably disposed outside thecenter between an end portion of the emitter layer 31 and an end portionof the contact hole 33 with respect to the longitudinal direction of theemitter layer 31. The emitter wiring line 34 may include the entireregion of the end regions 37 in plan view. The use of this configurationsuppresses an increase in the temperature of the end regions 37 andenables the effect of increasing the transition voltage Vt to beenhanced.

Next, a modification of the second embodiment will be described. In thesecond embodiment, the emitter electrode 32 is disposed inside theemitter layer 31. Alternatively, the emitter electrode 32 may extend tothe outside of the emitter layer 31 in plan view. In this case, therelative positional relationship between the emitter layer 31 and thecontact hole 33 is also the preferred relationship described above. Inthis modification, the emitter layer 31 is formed by self-alignment witha processing technique such as dry etching by using the emitterelectrode 32 as an etching mask. In this case, the emitter electrode 32disposed on the emitter layer 31 has a structure in which the emitterelectrode 32 slightly protrudes from edges of the emitter layer 31 andremains as an overhanging portion.

Third Embodiment

Next, a semiconductor device according to a third embodiment will bedescribed with reference to FIG. 16. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor devicesaccording to the first embodiment and the second embodiment will beomitted.

FIG. 16 is a plan view of an emitter layer 31, an emitter electrode 32,an ohmic contact interface 35, a contact hole 33, and an emitter wiringline 34 of a semiconductor device according to the third embodiment. Theemitter electrode 32 completely overlaps the ohmic contact interface 35in plan view. In the first embodiment, the distance a1 with respect tothe longitudinal direction is longer than the distance a2 with respectto the width direction. In the second embodiment, the distance b1 withrespect to the longitudinal direction is longer than the distance b2with respect to the width direction. In the third embodiment, both thedistance a1 and the distance b1 with respect to the longitudinaldirection are respectively longer than the distance a2 and the distanceb2 with respect to the width direction.

The comparison between FIG. 8B and FIG. 15B shows that a significanteffect of increasing the transition voltage Vt is obtained in the casewhere the distance a1 from an end portion of the emitter layer 31 to anend portion of the emitter electrode 32 with respect to the longitudinaldirection is increased, compared with the case where the distance b1from an end portion of the emitter layer 31 to an end portion of thecontact hole 33 with respect to the longitudinal direction is increased.However, an increase in the distance a1 decreases the area of the ohmiccontact interface 35, and thus there is a concern about a decrease inthe radio-frequency characteristics of the HBT. Accordingly, in FIG. 16,the distance a1 and the distance b1 with respect to the longitudinaldirection are preferably determined from the viewpoint of suppressing adecrease in the radio-frequency characteristics and the viewpoint ofincreasing the transition voltage Vt.

Fourth Embodiment

Next, a semiconductor device according to a fourth embodiment will bedescribed with reference to FIG. 17. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor deviceaccording to the first embodiment (FIGS. 4, 5, and 6) will be omitted.

FIG. 17 is a plan view of a semiconductor device according to the fourthembodiment. In the first embodiment, the distance a1 with respect to thelongitudinal direction is longer than the distance a2 with respect tothe width direction in both end portions of the emitter layer 31. In thefourth embodiment, the distance a1 with respect to the longitudinaldirection is longer than the distance a2 with respect to the widthdirection in one end portion of an emitter layer 31. In the other endportion, the distance a1 with respect to the longitudinal direction issubstantially equal to the distance a2 with respect to the widthdirection. The distance a1 with respect to the longitudinal direction ispreferably long in the end portion adjacent to a base electrode padportion 52B (the end portion on the left side in FIG. 17).

A current flowing in the emitter layer 31 is easily influenced bythermal and electrical effects from, for example, the base electrode padportion 52B, a contact hole 53, and a base wiring line 54 in the endportion on the left side compared with the end portion on the right sidein FIG. 17. The distance a1 with respect to the longitudinal directionis determined to be relatively long in an end portion of the emitterlayer 31, the end portion being more easily influenced by thermal andelectrical effects, to thereby offset the effects. Consequently, thekink K is unlikely to appear in the collector current-base voltagecharacteristics (FIG. 3). As a result, the effect of increasing thetransition voltage Vt is obtained. The distance a1 with respect to thelongitudinal direction in the end portion adjacent to the base electrodepad portion 52B is preferably 3 μm or more. Alternatively, the distancea1 with respect to the longitudinal direction in the end portionadjacent to the base electrode pad portion 52B is preferably 5 times ormore the distance a2 with respect to the width direction.

In the end portion on the side opposite to the contact hole 53, thedistance a1 with respect to the longitudinal direction is preferablyshorter than the distance a1 with respect to the longitudinal directionin the end portion adjacent to the base electrode pad portion 52B. Thedistance b1 with respect to the longitudinal direction in the endportion on the side opposite to the base electrode pad portion 52B isalso preferably shorter than the distance b1 with respect to thelongitudinal direction in the end portion adjacent to the base electrodepad portion 52B. For example, the distance a1 and the distance b1 withrespect to the longitudinal direction in the end portion on the sideopposite to the base electrode pad portion 52B are each preferably lessthan 1 μm.

The ohmic contact interface 35 in the fourth embodiment has a largerarea than the ohmic contact interface 35 in the first embodiment. As aresult, an HBT having good radio-frequency characteristics in ahigh-current range is obtained compared with the first embodiment.

Fifth Embodiment

Next, a semiconductor device according to a fifth embodiment will bedescribed with reference to FIG. 18. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor deviceaccording to the second embodiment (FIGS. 14A and 14B) will be omitted.

FIG. 18 is a plan view of a semiconductor device according to the fifthembodiment. In the second embodiment, the distance b1 with respect tothe longitudinal direction is longer than the distance b2 with respectto the width direction in both end portions of the emitter layer 31. Inthe fifth embodiment, the distance b1 with respect to the longitudinaldirection is longer than the distance b2 with respect to the widthdirection in one end portion of an emitter layer 31. In the other endportion, the distance b1 with respect to the longitudinal direction issubstantially equal to the distance b2 with respect to the widthdirection. In particular, the distance b1 with respect to thelongitudinal direction is preferably long in the end portion adjacent toa base electrode pad portion 52B (on the left side in FIG. 18).

In the fifth embodiment, the kink K (FIG. 3) is unlikely to appear inthe collector current-base voltage characteristics as in the secondembodiment. As a result, the effect of increasing the transition voltageVt is obtained. The distance b1 with respect to the longitudinaldirection in the end portion adjacent to the base electrode pad portion52B is preferably 4 μm or more. Alternatively, the distance b1 withrespect to the longitudinal direction in the end portion adjacent to thebase electrode pad portion 52B is preferably 5 times or more thedistance b2 with respect to the width direction. The distance b1 withrespect to the longitudinal direction in the end portion on the sideopposite to the base electrode pad portion 52B is preferably shorterthan the distance b1 with respect to the longitudinal direction in theend portion adjacent to the base electrode pad portion 52B. For example,the distance b1 with respect to the longitudinal direction in the endportion on the side opposite to the base electrode pad portion 52B ispreferably 1 μm or less. The distance a1 with respect to thelongitudinal direction is preferably 1 μm or less in both end portionsof the emitter layer 31.

Sixth Embodiment

Next, a semiconductor device according to a sixth embodiment will bedescribed with reference to FIG. 19. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor deviceaccording to the fourth embodiment (FIG. 17) will be omitted.

FIG. 19 is a plan view of a semiconductor device according to the sixthembodiment. In the fourth embodiment, the distance a1 with respect tothe longitudinal direction is longer than the distance a2 with respectto the width direction in the end portion adjacent to the base electrodepad portion 52B. In the sixth embodiment, instead of making the distancea1 with respect to the longitudinal direction long, an emitter electrode32 has a planar shape in which two corners of a rectangle are chamfered.

Oblique sides 39 formed by chamfering two adjacent corners of theemitter electrode 32 are continuous with each other, so that the emitterelectrode 32 has a pentagonal planar shape. The angle formed by each ofthe oblique sides 39 and the longitudinal direction of an emitter layer31 is, for example, 45°. The angle is not limited to 45°. The planarshape of an ohmic contact interface 35 completely overlaps the planarshape of the emitter electrode 32. The planar shape of a contact hole 33for an emitter also reflects the chamfering of the corners of theemitter electrode 32 and is a shape in which two corners of a rectangleare chamfered. A distance from an end portion of the emitter layer 31 inthe longitudinal direction to a farthest position of a chamfered portionwith respect to the longitudinal direction is referred to as a distancec. The distance c is preferably 3 μm or more as in the distance a1 ofthe fourth embodiment (FIG. 17).

In the sixth embodiment, the emitter access resistance in the chamferedportion of the emitter layer 31 is increased. Accordingly, the kink K(FIG. 3) is unlikely to appear in the collector current-base voltagecharacteristics as in the case of the fourth embodiment. As a result,the effect of increasing the transition voltage Vt to extend the rangeof the SOA is obtained. The ohmic contact interface 35 in the sixthembodiment has a larger area than the ohmic contact interface 35 in thefourth embodiment (FIG. 17). Therefore, a decrease in theradio-frequency characteristics can be suppressed.

Seventh Embodiment

Next, a semiconductor device according to a seventh embodiment will bedescribed with reference to FIG. 20. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor deviceaccording to the sixth embodiment (FIG. 19) will be omitted.

FIG. 20 is a plan view of a semiconductor device according to theseventh embodiment. In the sixth embodiment (FIG. 19), a base electrode52 includes two base electrode main portions 52A and a base electrodepad portion 52B that connects the base electrode main portions 52A toeach other as in the reference example (FIG. 1). In the seventhembodiment, a base electrode 52 incudes a single base electrode mainportion 52A and a base electrode pad portion 52B continuous with an endportion of the base electrode main portion 52A. The base electrode mainportion 52A is disposed on one side of an emitter layer 31 and extendsin a direction parallel to the longitudinal direction of the emitterlayer 31. The base electrode 52 including the base electrode mainportion 52A and the base electrode pad portion 52B has a planar shapesimilar to the form of the letter L.

In the sixth embodiment (FIG. 19), two corners in one end portion of theemitter electrode 32 are chamfered. In the seventh embodiment, only onecorner is chamfered. The chamfered corner is one that faces a cornerportion of the base electrode 52 having a shape of the letter L. Theangle formed by a chamfered oblique side 39 and the longitudinaldirection of the emitter layer 31 is, for example, 45°. The angle is notlimited to 45°.

In the seventh embodiment, the emitter access resistance in thechamfered portion of the emitter layer 31 is increased as in the sixthembodiment. Accordingly, the kink K (FIG. 3) is unlikely to appear inthe collector current-base voltage characteristics as in the case of thesixth embodiment. As a result, the effect of increasing the transitionvoltage Vt to extend the range of the SOA is obtained.

The distance c from an end portion of the emitter layer 31 in thelongitudinal direction to a farthest position of the chamfered portionwith respect to the longitudinal direction is preferably 3 μm or more asin the sixth embodiment. In the seventh embodiment, two corners in anend portion of an emitter electrode 32, the end portion being adjacentto the base electrode pad portion 52B, may be chamfered.

Eighth Embodiment

Next, a semiconductor device according to an eighth embodiment will bedescribed with reference to FIG. 21. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor deviceaccording to the seventh embodiment (FIG. 20) will be omitted.

FIG. 21 is a plan view of a semiconductor device according to the eighthembodiment. Two emitter layers 31 parallel to each other are disposed inthe eighth embodiment while one emitter layer 31 is disposed in theseventh embodiment (FIG. 20). An emitter electrode 32 and a contact hole33 are disposed so as to correspond to each of the two emitter layers31.

A base electrode main portion 52A is disposed between the two emitterlayers 31. A base electrode pad portion 52B is disposed at one endportion (the left end in FIG. 21) of the base electrode main portion52A. A base electrode 52 including the base electrode main portion 52Aand the base electrode pad portion 52B has a planar shape similar to theform of the letter T.

Each of the emitter electrodes 32 has a planar shape in which one cornerof a rectangle is chamfered as in the case of the seventh embodiment(FIG. 20). The chamfered corners are those that face a portion where thebase electrode main portion 52A and the base electrode pad portion 52Bare connected to each other.

In the eighth embodiment, the emitter access resistance in the chamferedportion of each of the emitter layers 31 is increased as in the seventhembodiment. Accordingly, the kink K (FIG. 3) is unlikely to appear inthe collector current-base voltage characteristics as in the case of theseventh embodiment. As a result, the effect of increasing the transitionvoltage Vt to extend the range of the SOA is obtained.

The distance c from an end portion of each of the emitter layers 31 inthe longitudinal direction to a farthest position of the chamferedportion with respect to the longitudinal direction is preferably 3 μm ormore as in the seventh embodiment. In the eighth embodiment, two cornersin an end portion of each of the emitter electrodes 32, the end portionbeing adjacent to the base electrode pad portion 52B, may be chamfered.

Ninth Embodiment

Next, a semiconductor device according to a ninth embodiment will bedescribed with reference to FIG. 22. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor deviceaccording to the sixth embodiment (FIG. 19) will be omitted.

FIG. 22 is a plan view of a semiconductor device according to the ninthembodiment. Two emitter layers 31 parallel to each other are disposed inthe ninth embodiment while one emitter layer 31 is disposed in the sixthembodiment (FIG. 19). An emitter electrode 32 and a contact hole 33 aredisposed so as to correspond to each of the two emitter layers 31.

Base electrode main portions 52A are disposed between the two emitterlayers 31 and outside each of the two emitter layers 31. A baseelectrode pad portion 52B connects the three base electrode mainportions 52A together. A base electrode 52 including the three baseelectrode main portions 52A and the base electrode pad portion 52B has aplanar shape similar to the form of the letter E. Each of the emitterelectrodes 32 has a planar shape in which two corners of a rectangle arechamfered as in the case of the sixth embodiment (FIG. 19).

In the ninth embodiment, the emitter access resistance in the chamferedportion of each of the emitter layers 31 is increased as in the sixthembodiment. Accordingly, the kink K (FIG. 3) is unlikely to appear inthe collector current-base voltage characteristics as in the case of thesixth embodiment. As a result, the effect of increasing the transitionvoltage Vt to extend the range of the SOA is obtained.

The distance c from an end portion of each of the emitter layers 31 inthe longitudinal direction to a farthest position of the chamferedportion with respect to the longitudinal direction is preferably 3 μm ormore as in the sixth embodiment.

Tenth Embodiment

Next, semiconductor devices according to a tenth embodiment will bedescribed with reference to FIGS. 23A, 23B, and 23C. Hereinafter,descriptions of configurations that are common to those of thesemiconductor device according to the first embodiment (FIG. 4) will beomitted.

FIGS. 23A, 23B, and 23C are plan views of an emitter layer 31, anemitter electrode 32, a contact hole 33, and an ohmic contact interface35 of semiconductor devices according to the tenth embodiment andmodifications of the tenth embodiment. In the first embodiment (FIG. 4),the emitter layer 31 has a rectangular planar shape. In contrast, in theexample illustrated in FIG. 23A, the emitter layer 31 has a planar shapein which an isosceles triangle is added to each of short sides on bothends of a rectangle, in other words, a long and narrow, hexagonal planarshape. In the example illustrated in FIG. 23B, the emitter layer 31 hasa planar shape in which four corners of a rectangle are rounded. In theexample illustrated in FIG. 23C, the emitter layer 31 has an octagonalplanar shape in which four corners of a rectangle are chamfered.

In each of the examples, the distance a1 with respect to thelongitudinal direction from an end portion of the emitter layer 31 inthe longitudinal direction to the ohmic contact interface 35 is longerthan the distance a2 with respect to the width direction. Therefore, thekink K (FIG. 3) is unlikely to appear in the collector current-basevoltage characteristics as in the case of the first embodiment. As aresult, the effect of increasing the transition voltage Vt to extend therange of the SOA is obtained.

Eleventh Embodiment

Next, a semiconductor device according to an eleventh embodiment will bedescribed with reference to FIG. 24. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor deviceaccording to the first embodiment will be omitted.

FIG. 24 is a plan view of a semiconductor device according to theeleventh embodiment. In the first embodiment, the emitter layer 31 (FIG.4) has a rectangular planar shape that is long in a direction parallelto one imaginary straight line. In the eleventh embodiment, an emitterlayer 31 has a planar shape similar to the form of the letter U formedby curving a rectangle. In this case, a direction along a center line ofthe curved rectangle (the circumferential direction of the curvedportion) can be defined as a longitudinal direction of the emitter layer31. A direction orthogonal to the longitudinal direction (the radialdirection of the curved portion) can be defined as a width direction ofthe emitter layer 31.

An emitter electrode 32 and a contact hole 33 also have planar shapessimilar to the form of the letter U, as in the emitter layer 31.

As in the case of the first embodiment, a gap between an edge of theemitter layer 31, the edge being located at an end portion in thelongitudinal direction of the emitter layer 31, and an edge of theemitter electrode 32, the edge being located at an end portion in thelongitudinal direction of the emitter electrode 32, is referred to as adistance a1 with respect to the longitudinal direction. A gap between anedge along the longitudinal direction (an edge of a curved portion) ofthe emitter layer 31 and an edge along the longitudinal direction (anedge of a curved portion) of the emitter electrode 32 is referred to asa distance a2 with respect to the width direction. The distance a2 withrespect to the width direction in the curved portion corresponds to, forexample, a distance in the radial direction between an edge of theemitter layer 31 and an edge of the emitter electrode 32 on the outercircumferential side or a distance in the radial direction between anedge of the emitter layer 31 and an edge of the emitter electrode 32 onthe inner circumferential side.

An emitter wiring line 34 is disposed so as to overlap the emitterelectrode 32. The emitter wiring line 34 is connected to the emitterelectrode 32 through the contact hole 33.

A base electrode 52 is disposed in a region surrounded by the U-shapedemitter layer 31. A base wiring line 54 is disposed so as to overlap thebase electrode 52. The base wiring line 54 is connected to the baseelectrode 52 through a contact hole 53.

A collector electrode 42 is disposed so as to surround the emitter layer31 from the outside of the curved portion. A collector wiring line 44 isdisposed so as to overlap the collector electrode 42. The collectorwiring line 44 is connected to the collector electrode 42 through acontact hole 43.

In the eleventh embodiment, the distance a1 with respect to thelongitudinal direction of the emitter layer 31 is longer than thedistance a2 with respect to the width direction as in the case of thefirst embodiment. Therefore, the kink K (FIG. 3) is unlikely to appearin the collector current-base voltage characteristics as in the case ofthe first embodiment. As a result, the effect of increasing thetransition voltage Vt to extend the range of the SOA is obtained. Thedistance a1 with respect to the longitudinal direction of the emitterlayer 31 is preferably 3 μm or more as in the case of the firstembodiment.

Twelfth Embodiment

Next, a semiconductor device according to a twelfth embodiment will bedescribed with reference to FIG. 25. Hereinafter, descriptions ofconfigurations that are common to those of the semiconductor deviceaccording to the sixth embodiment (FIG. 19) will be omitted.

FIG. 25 is a plan view of a semiconductor device according to thetwelfth embodiment. In the sixth embodiment (FIG. 19), the emitterelectrode 32 has a planar shape in which corners of a rectangle arechamfered. In the twelfth embodiment, a contact hole 33 for an emitterhas a planar shape in which corners of a rectangle are chamfered. Anemitter electrode 32 and an ohmic contact interface 35 each have arectangular planar shape.

In the twelfth embodiment, a current flowing in an emitter layer 31located right under the chamfered portion of the contact hole 33 isdecreased as in the case of the second embodiment (refer to FIGS. 14A to15B). Therefore, the kink K (FIG. 3) is unlikely to appear in thecollector current-base voltage characteristics as in the case of thesecond embodiment. As a result, the effect of increasing the transitionvoltage Vt to extend the range of the SOA is obtained.

A distance from an end portion of the emitter layer 31 in thelongitudinal direction to a farthest position of the chamfered portionwith respect to the longitudinal direction is referred to as a distanced. The distance d is preferably 4 μm or more as in the case of thesecond embodiment.

Next, modifications of the twelfth embodiment will be described. In thetwelfth embodiment, corners of the contact hole 33 for the emitter arechamfered instead of chamfering corners of the emitter electrode 32(FIG. 19) of the sixth embodiment. At least one corner of the contacthole 33 for an emitter may be chamfered instead of chamfering at leastone corner of the emitter electrode 32 of the semiconductor deviceaccording to the seventh embodiment (FIG. 20), the eighth embodiment(FIG. 21), or the ninth embodiment (FIG. 22). In such a case, thedistance d is preferably 4 μm or more as in the case of the twelfthembodiment.

The embodiments and modifications described above are exemplary, and,needless to say, a partial replacement or combination of configurationsdescribed in different embodiments and modifications is possible. Thesame or similar operations and effects achieved by the same or similarconfigurations in a plurality of embodiments and modifications will notbe mentioned in each of the embodiments. Furthermore, the presentdisclosure is not limited to the embodiments and modifications describedabove. For example, it is obvious for those skilled in the art thatvarious changes, improvements, combinations, and the like can be made.

While preferred embodiments of the disclosure have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the disclosure. The scope of the disclosure, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A semiconductor device comprising: a collectorlayer, a base layer, and an emitter layer that are disposed on asubstrate to form a bipolar transistor; an emitter electrode that is inohmic contact with the emitter layer; and an emitter wiring lineconnected to the emitter electrode through a contact hole formed in aninsulating film, wherein the emitter layer has a shape that is long inone direction in plan view, and a difference in dimension with respectto a longitudinal direction of the emitter layer between the emitterlayer and the contact hole is larger than a difference in dimension withrespect to a width direction of the emitter layer between the emitterlayer and the contact hole.
 2. The semiconductor device according toclaim 1, wherein the difference in dimension with respect to thelongitudinal direction of the emitter layer between the emitter layerand the contact hole is 10 times or more the difference in dimensionwith respect to the width direction of the emitter layer between theemitter layer and the contact hole.
 3. The semiconductor deviceaccording to claim 2, wherein a distance from an end portion of theemitter layer in the longitudinal direction to the contact hole withrespect to the longitudinal direction of the emitter layer is 5 times ormore the difference in dimension with respect to the width direction ofthe emitter layer between the emitter layer and the contact hole in bothend portions of the emitter layer in the longitudinal direction.
 4. Thesemiconductor device according to claim 1, wherein a distance from anend portion of the emitter layer in the longitudinal direction to thecontact hole with respect to the longitudinal direction of the emitterlayer is 4 μm or more in at least one end portion of the emitter layerin the longitudinal direction.
 5. The semiconductor device according toclaim 2, wherein a distance from an end portion of the emitter layer inthe longitudinal direction to the contact hole with respect to thelongitudinal direction of the emitter layer is 4 μm or more in at leastone end portion of the emitter layer in the longitudinal direction. 6.The semiconductor device according to claim 3, wherein the distance fromthe end portion of the emitter layer in the longitudinal direction tothe contact hole with respect to the longitudinal direction of theemitter layer is 4 μm or more in at least one end portion of the emitterlayer in the longitudinal direction.
 7. The semiconductor deviceaccording to claim 1, wherein a difference in dimension with respect tothe longitudinal direction of the emitter layer between the emitterlayer and an ohmic contact interface at which the emitter layer and theemitter electrode are in ohmic contact with each other is larger than adifference in dimension with respect to the width direction of theemitter layer between the emitter layer and the ohmic contact interface.8. The semiconductor device according to claim 7, wherein the differencein dimension with respect to the longitudinal direction of the emitterlayer between the emitter layer and the contact hole is 10 times or morethe difference in dimension with respect to the width direction of theemitter layer between the emitter layer and the contact hole.
 9. Thesemiconductor device according to claim 7, wherein a distance from anend portion of the emitter layer in the longitudinal direction to thecontact hole with respect to the longitudinal direction of the emitterlayer is 4 μm or more in at least one end portion of the emitter layerin the longitudinal direction.
 10. A semiconductor device comprising: acollector layer, a base layer, and an emitter layer that are disposed ona substrate to form a bipolar transistor; and an emitter electrode thatis in ohmic contact with the emitter layer, wherein the emitter layerhas a shape that is long in one direction in plan view, and an ohmiccontact interface at which the emitter layer and the emitter electrodeare in ohmic contact with each other has a planar shape in which atleast one corner of a rectangle is chamfered.
 11. The semiconductordevice according to claim 10, wherein a distance from an end portion ofthe emitter layer in a longitudinal direction to a farthest position ofa chamfered portion of the ohmic contact interface with respect to thelongitudinal direction of the emitter layer is 3 μm or more.
 12. Asemiconductor device comprising: a collector layer, a base layer, and anemitter layer that are disposed on a substrate to form a bipolartransistor; an emitter electrode that is in ohmic contact with theemitter layer; and an emitter wiring line connected to the emitterelectrode through a contact hole formed in an insulating film, whereinthe emitter layer has a shape that is long in one direction in planview, and the contact hole has a planar shape in which at least onecorner of a rectangle is chamfered.
 13. The semiconductor deviceaccording to claim 12, wherein a distance from an end portion of theemitter layer in a longitudinal direction to a farthest position of achamfered portion of the contact hole with respect to the longitudinaldirection of the emitter layer is 4 μm or more.